Photophysics of PTCDA and Me-PTCDI thin films: effects of growth temperature.

The effect of deposition temperature on the photophysical properties of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,N'-dimethylperylene-3,4,9,10-bis(dicarboximide) (Me-PTCDI) films is investigated with steady-state and time-resolved spectroscopy. Atomic force microscopy (AFM) images of the film surfaces show an increase in the dimensions of crystallites with substrate temperature, culminating in the formation of elongated crystallites on substrates held close to the sublimation temperature. In contrast, despite an improvement in the crystal quality, X-ray diffraction (XRD) studies indicate that the substrate temperature has a negligible effect on the molecular orientation; the PTCDA and Me-PTCDI molecules align parallel and tilted to the substrate surface, respectively. Both materials exhibit characteristic absorption, due to mixing between charge-transfer and Frenkel species, and broad structureless photoluminescence. Growth at elevated temperatures gives rise to increased low-energy absorption, attributed to the formation of charge-transfer species, and enhanced blue-shifted emission, although the effects are less pronounced for Me-PTCDI. Time-correlated single-photon counting data indicate that the enhancement coincides with a lengthening of the fluorescence decays, over the whole emission spectrum.